Serial schemes eyed for disk storage
By Martin Czekalski, Technical Marketing Manager, Maxtor Corp., Director and Treasurer, SCSI Trade Association, San Francisco, EE Times
October 28, 2002 (10:19 a.m. EST)
The current state of the industry for device-level storage attachment requires system designers to develop multiple solutions for a full range of storage price/performance options. With the advent of Serial Attached SCSI, designers can use a common infrastructure to attach Serial Attached SCSI disks for performance-oriented, enterprise-class applications, or Serial ATA (SATA) disks for cost-optimized applications, or both.
Parallel interfaces face challenges, including performance, device addressability, signal integrity, cabling distance, incompatibility with new silicon processes and large connectors unsuitable for dense computing environments. With their increasing cost and complexity, parallel interfaces are no longer the optimum way to attach storage and other devices.
Today, serial technology is recognized by many engineers as the future for attached storage devices, meeting needs for longer distances, faster data throughput, small er connectors, and simplified cable and backplane routing. The migration to serial technology has already occurred at the box-to-box level with Fibre Channel and Ethernet, as well as at the device level with Fibre Channel arbitrated loop (FC-AL).
However, it is the advent of SATA and Serial Attached SCSI that will offer system designers the opportunity to develop a flexible, yet cost-effective, point-to-point interconnect solution.
With the support of a variety of industry organizations and the backing of a significant number of large corporations, the migration to serial storage interfaces is gathering momentum. With the SATA 1.0 specification now released, many companies are beginning to migrate to this new serial interface, and a number of products are currently available.
The Serial Attached SCSI Working Group, an independent industry consortium of storage, server and hard-drive vendors, came together to draft a preliminary Serial Attached SCSI interface specification.
In May, this specification was accepted by the T10 committee, the official SCSI standards group of the International Committee for Information Technology Standards, as a working draft of the Serial Attached SCSI standard. Since then, work has continued with the goal of completing the specification for letter ballot by the end of the year.
In addition, the SCSI Trade Association (STA), a member-run industry group that has taken responsibility for Serial Attached SCSI, is working to define marketing requirements for the Serial Attached SCSI specification and to promote the technology to the industry. The STA estimates that Serial Attached SCSI products will enter the market in early 2004.
Serial Attached SCSI will support internal and near-attached storage devices. At the same time, it addresses the storage market's future needs by giving system engineers the chance to develop a common infrastructure that supports enterprise-class Serial Attached SCSI and desktop-class SATA drives.
Both SATA and Serial Attached SCSI drives can be physically inserted on the same backplane connector. At the protocol level, the subsystem will use SATA Tunneling Protocol for SATA devices and Serial SCSI Protocol for Serial Attached SCSI devices as required.
Serial Attached SCSI defines a switching architecture for connecting multiple drives to multiple ports. Low-cost expanders allow designers to maintain a point-to-point architecture while connecting multiple drives. Devices can be easily swapped and redeployed as needed, with minimal disruption for users.
Serial Attached SCSI offers backward compatibility with SCSI software and middleware, and leverages existing technology and standards to reduce risk and time-to-market. Its point-to-point switched architecture provides wide links for performance as well as expansion and connectivity. This scalability and expan dability is a substantial benefit over the shared-media approach of existing interconnects.
From a reliability perspective, both Serial Attached SCSI and SATA will provide solutions consistent with their parallel counterparts. When cost is the primary design consideration, a SATA configuration is indicated, providing a bounded solution and modest performance at a modest price. For mission-critical applications that require increased availability, scalability and performance, Serial Attached SCSI is the answer. Its single infrastructure provides access to both multiple hosts and multiple paths, serving a broad range of applications and uses. Mixed pools of storage can meet cost objectives with high-capacity SATA drives for near-line devices, archival storage and backup, as well as high-performance Serial Attached SCSI drives for transaction-based applications.
Fibre Channel will continue to evolve for use in very high-end large-enterprise storage configurations, offering 16 million addresses (public loop) and longer cabling distances appropriate for complex SAN applications. Bridging between a Fibre Channel SAN and Serial Attached SCSI configuration is a straightforward design process, since they share the same SCSI packet formats. In configurations where long distance and public loop are not needed, consider using Serial Attached SCSI as a device-level interconnect.